Thermal recording material

Abstract: A thermal recording material comprises a support coated with a colour developing layer, which colour developing layer comprises (i) a developer which is a bisthiourea compound, for example of formula (A-2): and (ii) a colourless dye precursor with which the developer reacts upon heating to form a colour. Typically the thermal colour developing layer contains no sensitizer. In the thermal recording materials of the present invention, substantially no colour is developed at a temperature of 120°C while colour development can be caused in response to heating with a thermal head or a laser beam. The developed colour on the material can be erased upon contacting with alcoholic solvents, depending on the thiourea compound contained in the colour developing layer. After erasure of the image, the material can be used again for recording with a thermal head or a laser beam.

Description: The present invention relates to a thermal recording material. Typical thermal recording materials comprise a support such as a sheet of paper, synthesized paper, film or plastic. The support is coated with a coating solution. The coating solution comprises an electron-donating, colorless dye precursor which is normally colorless or light-colored, and an electron-accepting developer such as a phenolic compound. The dye precursor and the developer are separately ground into fine particles and mixed together, to which a binder, a filler, a sensitizer, a lubricant and other additives are added. In response to heating through a thermal head, a thermal pen, a laser beam or the like, the dye precursor instantaneously reacts with the developer to provide visible recording images. Such thermal recording materials have been applied in a wide range of fields including measuring recorders, printers for computer terminals, facsimile devices, automatic ticket vending machines, bar-code labels, etc. The quality requirement of the consumer for the thermal recording materials has been sophisticated as the recording devices have been diversified and had higher performances. For example, it has been required to (a) offer high-density and clear developed images with a smaller amount of thermal energy to increase recording speed and (b) have good storability involving light resistance, heat resistance, water resistance, oil resistance, and plasticizer resistance. There are now more opportunities than before for comparing thermal papers with plain papers, with increasing use of methods of recording data on plain paper such as electrophotographic methods and ink-jet recording methods. For instance, preservability of the resultant images on a thermal recording material is required to have comparable quality to those recorded with a toner. On the other hand, preservability of non-recorded portions (background portions of the paper), hereinafter, referred to as background preservability, is required to be close in quality to plain paper. A background preservability against heat (100°C or higher) or plasticizers is particularly required. For the background preservability against heat, JP-A-4-353490 (the term "JP-A" as used herein means Japanese Patent Laid-open, or an "unexamined" published Japanese patent application) discloses a thermal recording material in which background or whiteness and density of recorded portion do not deteriorate in a high temperature environment of around 90°C. More specifically, the background of this thermal recording material has a density of some 0.11, measured by a Macbeth densitometer, after processing in a drier at 95°C for 5 hours. This result is relatively fair but is not of a satisfactory level. In addition, conventional recording materials employing a phenolic developer possess insufficient heat resistance, so that it is impossible to laminate a film or the like by heat sealing or thermal laminating on the surface of the material subjected to the thermal recording. The background preservability can be improved by using thermal materials comprising a thiourea compound rather than a phenolic one. The thiourea compound differs in structure from the phenolic compounds commonly used as developer. For example, JP-A-58-211496, JP-A-59-184694, JP-A-60-145884, JP-A-61-211085, JP-A-5-4449, and JP-A-5-185739 disclose thermal recording materials comprising a thiourea compound as developer, which materials have superior background preservability (such as heat resistance, water resistance, and plasticizer resistance) and preservability of the recorded images. Of these, the thiourea compounds disclosed in JP-A-58-211496, JP-A-59-184694, and JP-A-61-211085 are monourea compounds having only one phenylthiourea moiety Ar-NH-C=S-NH. These compounds are not superior to the phenolic developer in view of heat resistance, and this is a major cause of their unsuccessful use in practical applications. On the other hand, JP-A-60-145884 discloses diphenyl-bis-thiourea, diphenyl- p -phenylene-dithiourea, and diphenyl- m -phenylene-dithiourea. The first of these has two phenylthiourea moieties which are directly linked to each other. The remaining two each have two phenylthiourea moieties linked through a phenylene group. These thiourea compounds are, however, used along with a color developing enhancer and thus have poor heat resistance. In addition, these thiourea compounds have certain disadvantages in the heat resistance of the recorded and background portions even when used alone. JP-A-5-185739 discloses a thermal recording material in which a bisthiourea compound is used as the developer to provide improved resistance to ethanol and plasticizers. The bisthiourea compound is used, however, along with a sensitizer. Accordingly, it is impossible to achieve heat resistance at 100°C or higher. JP-A-5-4449 discloses that recorded images can be stabilized by means of adding, as a third compound, a bisthiourea compound to the color developing composition comprising a dye precursor and a salicylic acid developer. However, this compound can provide neither heat resistance at 100°C or higher nor a "reversible recording" characteristic, which allows a repeated cycle of recording and erasing, when being prepared according to a method disclosed in the specification. Reversible recording is an important factor in the field of current thermal recording. More specifically, tremendous efforts have been made to improve thermal recording materials and such improvements result in a rapid increase in the level of consumption of recording materials. This means that the volume of thermal recording papers used has increased, which contributes to current social problems of refuse disposal. A thermal reversible recording material that permits recording and erasing repeatedly has thus been of interest as an approach to this problem. For example, JP-A-3-230993, and JP-A-4-366682 disclose thermal reversible recording materials whose state changes reversibly from transparent to opaque as a function of temperature. The recording materials for opaque appearance are, however, inferior in clarity and brightness. In addition, color recording is not available in some of the thermal reversible recording materials. An object of the present invention is to provide a thermal recording material having an improved heat resistance. Another object of the present invention is to provide a reversible recording material whose state changes reversibly. Accordingly, the present invention provides a thermal recording material comprising a support coated with a color developing layer, wherein the color developing layer comprises: (i) a developer which comprises a bisthiourea compound of the general formula (I): wherein each of X, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , and R 8 , which may be the same or different, is a lower alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, a cyclohexyl group, a nitro group, a cyano group, a halogen atom or a hydrogen atom; Y is S or SO 2 ; and m is an integer of from 1 to 3; and (ii) a colourless dye precursor with which the developer reacts upon heating to form a colour. The recorded portion is erased with an alcoholic solvent without affecting preservability and stability of a background. The thermal recording material according to the present invention is excellent in the heat resistance. This means that the thermal recording material can be subjected to heat sealing or thermal laminating after an image is recorded thereon. The developer used in the present invention is a conventional bisthiourea compound selected by means of producing thermal recording materials and performing tests on thermal laminating and heat resistance with heat rolls. [Production of Thermal Recording Materials] Thermal recording materials were produced with bisthiourea compounds used as the developers and 3-N,N-diethylamino-6-methyl-7-anilinofluoran (ODB) used as the dye precursor. The formulation was as follows: (Dispersion of Developer) Bisthiourea Compound 6.0 parts by weight 10%-polyvinyl Alcohol 18.8 parts by weight Water 11.2 parts by weight (Dispersion of Dye Precursor) ODB 2.0 parts by weight 10%-polyvinyl Alcohol 4.6 parts by weight Water 2.6 parts by weight 36.0 parts by weight of the developer dispersion, 9.2 parts by weight of the dye precursor dispersion and 12.0 parts by weight of 50%-dispersion of kaolin clay were mixed into a coating solution. This solution was coated on one surface of a paper support of 50 g/m 2 in a coating amount of 6.0 g/m 2 , which was then subjected to super-calendering to produce a thermal recording material with a smoothness of 500-600 seconds. [Thermal Laminating Test] The thermal recording materials so produced were subjected to thermal printing with a word processor to cause color development, following which the materials were subjected to thermal laminating with a simple laminating machine. Subsequently, color-developed and background portions of the materials were measured with a Macbeth densitometer. [Heat Resistance Test] The thermal recording materials so produced were forced to a hot plate, which had previously heated to 200°C, at a pressure of 10 g/cm 2 for 5 seconds to cause color development. The color-developed thermal recording materials were passed between heat rolls of 160°C at a speed of 30 mm/s. Subsequently, color-developed and background portions of the materials were measured with a Macbeth densitometer. Specific examples of the bisthiourea compound of the general formula (I) include the following compounds. The thermal recording materials comprising the bisthiourea compound of the general formula (I) exhibited excellent heat resistance in the above mentioned heat resistance test. The "thermal recording material having the reversible recordability" is the one whose state changes reversibly. More specifically, recorded portion on the surface of the material can be erased by means of contacting the surface with an alcoholic solvent. The resultant material can be used for re-recording other images with a thermal head or a laser beam. Exemplified alcoholic solvents are: methanol, ethanol, n -propylalcohol, iso -propylalcohol, n -butanol, sec -butanol, and tert -butanol. It is apparent that the background should be stable during erasing with the alcoholic solvent. Alternatively, recorded images on the thermal recording material comprising a specific developer may be erased by means of transferring a certain level of a thermal energy to the surface of the material with, for example, heat rolls, thermal-head, drying oven. After erasing, the thermal recording material can be recycled for another recording. For example, the thermal recording material comprising the compound A-1 exhibited good erasability when being passed between heat rolls. The idea of the present invention to change the state of the thermal recording material reversibly is quite the opposite to the viewpoint of ethanol resistance disclosed in JP-A-5-185739. More specifically, this conventional thermal recording material comprises 2-anilino-3-methyl-6-(N-ethyl-N-tetrahydrofurfurylamino)fluoran as the dye, a bisthiourea compound as the developer, and di( p -methylbenzyl) oxalate as the sensitizer. JP-A-5-185739 discloses resistance to ethanol and plasticisers. On the contrary, in the present invention, the bisthiourea compounds are selected according to erasability or discoloration of the recorded portion with ethanol. The thermal recording materials whose state changes reversibly are preferably capable of providing low preservability for the recorded portion and high preservability for the background. To achieve this feature, a sensitizer may advantageously be used. As mentioned above, sensitizers badly affect the heat resistance but are favorable in reversible recording. Preferred examples of the sensitizer applicable for this purpose include: 2-di(3-methylphenoxy)ethane, p -benzylbiphenyl, β-benzyloxynaphtalene, 4-biphenyl- p -tolylether, m -terphenyl,1,2-diphenoxyethane, dibenzyl oxalate, and di( p -chlorobenzyl) oxalate. The thermal recording material according to the present invention, which comprises the bisthiourea compound and which has excellent heat resistance, has a "conflicting" feature that substantially no color is developed at a temperature of 120°C while color development can be caused with a thermal head or the like. The developed color on the thermal recording material of the present invention will not be erased or discolored when the material contacts with organic solvents other than alcoholic ones. Likewise, the background is not changed in color upon contacting with organic solvents other than alcoholic ones. This may be because the bisthiourea compounds used in the present invention have low solubility to such organic solvents. The thermal recording materials having excellent heat resistance according to the present invention have another advantage of easy management of manufacturing process. Typical methods of manufacturing thermal recording materials include the process of drying the thermal color developing layer after the coating solution has been applied on the surface of a support. Conventional drying should take place under strict temperature control to avoid color development of the background on the coated surface. This restricts the coating speeds which can be used. On the contrary, in the thermal recording material of the present invention no color develops on the background when the material is exposed to hot air of 110°C in a dried environment. This permits the drying process to take place at a high temperature. In addition, the range of suitable drying temperatures increases with an associated probable rapid increase of productivity. As mentioned above, in the thermal reversible recording material of the present invention, the recorded portion on the surface of the material can be erased by contacting the surface with an alcoholic solvent. Alternatively, the images may be erased by transferring a certain level of thermal energy to the surface of the material with, for example, heat rolls or the like in adequate thermal conditions, depending on the compound contained in the color developing layer. After erasing, the material can be used for re-recording of other images with a thermal head or a laser beam. The thermal recording materials according to the present invention are manufactured by any conventional method comprising preparing a coating solution, coating the solution on a support, and drying the solution. The coating solution may be prepared by dispersing (a) a dye precursor, and (b) a bisthiourea compound, which serves as a developer separately with a binder. The coating solution may further contain one or more additives such as fillers, lubricants, ultraviolet ray absorbers, water-proof agents, and anti-foaming agents. The dye precursor used in the thermal recording material of the present invention is not limited and may be any conventional dye precursor known in the field of thermal recording. However, it is preferable to use a triphenylmethane-, fluoran-, or fluoren-based dye. Preferable examples of the dye precursor are given below. 〈Triphenylmethane Leuco Dyes〉 Crystal Violet Lactone (CVL), and Malachite Green Lactone (MGL) 〈Fluoran Leuco Dyes〉 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-( o,p -dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-( m -trifluoromethylanilino)fluoran, 3-diethylamino-6-methyl-7-( o -chloroanilino)fluoran, 3-diethylamino-6-methyl-chlorofluoran, 3-diethylamino-6-methyl-fluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-ethoxyethyl-7-anilinofluoran, 3-diethylamino-benzo[a]-fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-( o,p -dimethylanilino)fluoran, 3-dibutylamino-6-methyl-7-( m -trifluoromethylanilino)fluoran, 3-dibutylamino-6-methyl-7-( o -chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-( o -fluoroanilino)fluoran, 3-dibutylamino-6-methyl-chlorofluoran, 3-dibutylamino-6-methyl-fluoran, 3-dibutylamino-6-chloro-7-anilinofluoran, 3-di n -pentylamino-6-methyl-7-anilinofluoran, 3-di n -pentylamino-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-hexylamino-6-methyl-7-( p -chloroanilino)fluoran, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-cyclohexylamino-6-chlorofluoran, 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)-3-diethylamino-6-methyl-7-anilinofluoran, and 2-(4-oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran 〈Fluoren Leuco Dyes〉 3,6,6'-tris(dimethylamino)spiro[fluoren-9-3'-phthalid], and 3,6,6'-tris(diethylamino)spiro[fluoren-9-3'-phthalid] These dye precursors may be used alone or may be a mixture of two or more dye precursors. The fluoran dye precursors can be used advantageously in the present invention because the thermal recording material comprising the precursor of this type can provide improved preservability of the background at high temperature. When the thermal stability or preservability of the background is an important factor, it is preferable to use a dye having a high melting point and a high decomposition temperature. In addition, it is preferable to mix two or more kinds of dye precursors. On the contrary, when a reversible recordability is an important factor, a dye such as 3-diethylamino-7-(m-trifluoromethylanilino)fluoran is particularly preferable. Examples of the binder applicable to the present invention include: completely silicified polyvinyl alcohol, partially saponified polyvinyl alcohols, carboxy denatured polyvinyl alcohols, amides denatured polyvinyl alcohols, sulfonic acid denatured polyvinyl alcohols, butylal denatured polyvinyl alcohols, other denatured polyvinyl alcohols, which are each 200-1,900 in degree of polymerization (D.P.); cellulose derivatives such as hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, ethyl cellulose and acetyl celluloses, styrene-maleic anhydride copolymers, styrene-butadiene copolymers; polyvinyl chloride, polyvinyl acetate, polyacrylamide, polyester acrylate, polyvinyl butylal, polystyrol, and copolymers thereof; polyamide resins, silicon resins, petroleum resins, terpene resins, ketone resins, and chroman resins. Of these, polyvinyl alcohol binders are preferable by the considerations of dispersability, binding capacities, and thermal stability of the background. These binders may be dissolved in a solvent such as water, alcohols, ketones, esters, and hydrocarbons. Alternatively, the binders may be dispersed in water or other medium as an emulsion or paste. In addition, a combination of dissolution and dispersion may be used depending on the quality being required. Examples of the filler applicable to the present invention include: inorganic fillers such as silica, calcium carbonate, kaolin, diatomaceous earth, talc, titanium oxide, and aluminum hydroxide; and organic fillers such as organic polystyrene fillers, organic styrene-butadiene fillers, and organic styrene-acryl fillers. In addition to the above mentioned additives, other compounds may be added such as parting agents like fatty acid metal salts, lubricants like waxes, benzophenone-based or benzotriazole-based ultraviolet ray absorbers, water-proof agents like glyoxal, dispersants, and anti-foaming agents. There are no limitations on amounts of the bisthiourea compound and the dye precursor blended in a coating solution according to the present invention, the kind of the other components, and the amounts thereof. Instead, they are determined according to the desired performances and recording aptitudes. However, a simple blending is preferable to avoid deterioration of the thermal stability of the background. In a typical example, the coating solution comprises 1 to 8 parts by weight of bisthiourea developer and 1 to 20 parts by weight of filler relative to 1 part of dye precursor. In addition, the coating solution comprises 10% to 25% by weight of binder, based on the total weight of solids. These compounds are formed into fine particles having a particle diameter of several microns or smaller through a grinder such as a ball mill, an attritor, and a sand grinder or any other emulsifying machines. The binder and other additives, if necessary, are added to the fine particles, which is then prepared into the coating solution. The coating solution having the above mentioned composition is applied to an adequate support to provide a desired thermal recording material. The support may be a sheet of paper or synthetic paper, an unwoven fabric, a metal foil, a plastic film, a plastic sheet, or a combination thereof as a composite sheet. The thermal recording material so obtained can be provided with an overcoating layer on the thermal color developing layer to improve the preservability or storability. Alternatively, an undercoating layer may be provided under the thermal recording layer to improve color developing sensitivity. The overcoating layer may be a polymer material while the undercoating layer may be a polymer material containing one or more fillers. In particular, the thermal recording material according to the present invention which is excellent in the heat resistance can be provided with a transparent, strong protecting coating by means of thermal laminating a film on the surface of the material having images recorded thereon using the high thermal stability of the background. In this event, commercially available simple laminating machines may be used to make through a simple manner a card with the thermal-recorded images thereon. It has not yet been elucidated why the bisthiourea compounds alone can serve as the developers for the dye precursors, why the high heat resistance which is not expected before can be achieved by means of eliminating sensitizers, and why the high reversible recordability can be achieved depending on the compounds. However, a probable reason for these points is that the thiourea compounds according to the present invention are changed in structure from thioketon to enthiol or vise versa as given below: It is expected that enthiolation is essential for the bisthiourea compounds to function as the developer. Enthiolation can occur only at a high temperature. With a thermal head, a high temperature of from 200° to 300°C is achieved instantaneously, so that the bisthiourea compound contacting with the thermal head is enthiolated, which results in color developing capability to break a lactone ring of the dye precursor and hence to develop the color. On the other hand, the bisthiourea compound is not changed at a temperature lower than that causing enthiolation. Accordingly, the bisthiourea compound is not reacted with the dye precursor and the background remains white. This may explain the high heat resistance of the materials according to the present invention. In addition, a good color developing feature cannot be achieved with the monothiourea compounds probably because they have only one active hydrogen. On the contrary, the bisthiourea compounds have the increased number of active hydrogens, which may contribute to achieving the good color developing features. The thermal recording materials of this invention are also excellent in resistance to solvents. This may be because the bisthiourea compounds have an extremely low solubility to the solvents, and substantially no developer is mixed with the dye precursor upon contacting with the solvents. Finally, discoloration may occur to provide the reversible recordability when the thermal recording material is changed in structure from enthiol back to thioketon due to a certain reason. This thioketonation may be caused upon contacting with alcoholic solvents, otherwise with adequate temperature and thermal energy. Enthiolation and thioketonation occur under different conditions, so that the thermal recording material can be changed in structure repeatedly between enthiol and thioketon, which permits the reversible recording. The foregoing features of the present invention will be more readily apparent in the context of a specifically delineated set of examples and controls. However, it should be understood that the present invention is not limited to those particular examples and the reference as long as it does not depart from the spirit and scope of the appended claims. In the following description, all percents and parts are by weight unless otherwise specified. [EXAMPLES 1-6] Thermal recording materials were produced with bisthiourea compounds used as the developers and 3-N,N-diethylamino-6-methyl-7-anilinofluoran (ODB) used as the dye precursor. The formulation was as follows. (Dispersion of Developer) Bisthiourea Compound (See, Table 1) 6.0 parts Aqueous Solution of 10%-polyvinyl Alcohol 18.8 parts Water 11.2 parts (Dispersion of Dye Precursor) ODB 2.0 parts Aqueous Solution of 10%-polyvinyl Alcohol 4.6 parts Water 2.6 parts. Each dispersion of the above mentioned compounds were ground into fine particles having an average particle diameter of 1 micron by using a sand grinder. Subsequently, the dispersions were mixed in a following formulation to prepare a coating solution. Dispersion of Developer 36.0 parts Dispersion of Dye Precursor 9.2 parts Kaolin Clay (50% dispersion) 12.0 parts This solution was coated on one surface of a paper support of 50 g/m 2 in a coating amount of 6.0 g/m 2 , which was then subjected to super-calendering to produce a thermal recording material with a smoothness of 500-600 seconds. [Controls 1-17] Thermal recording materials were produced for comparison with following known compounds used as the developer: bisphenol A (E-1), bisphenol S (E-2), 4-hydroxy-4'- iso -propoxydiphenylsulfon (E-3), 4-hydroxy-4'- n -butoxydiphenylsulfon (E-4), 1,3-diphenylthiourea (E-5) (disclosed in JP-A-58-211496), 1,3-benzylphenylthiourea (E-6), 1,3-phenylstearylthiourea (E-7), 1,3-di( m -chlorophenyl)thiourea (E-8), 1,3-di( p -toluyl)thiourea (E-9) (disclosed in JP-A-58-211496), diphenylbisthiourea (E-10) (disclosed in JP-A-60-145884), bisthiourea compound (E-11) (disclosed in JP-A-5-185739), bisthiourea compound (E-12) (disclosed in JP-A-5-185739), bisthiourea compound (E-13) (disclosed in JP-A-5-185739), diphenyl- p -phenylene-dithiourea (E-14) (disclosed in JP-A-60-145884), diphenyl- m -phenylene-dithiourea (E-15) (disclosed in JP-A-60-145884), bisthiourea compound (E-16) (disclosed in JP-A-5-185739), and bisthiourea compound (E-17) (disclosed in JP-A-5-185739). (Dispersion of Developer) Comparative Compound (E-1 through E-17) (See, Table 2) 6.0 parts Aqueous Solution of 10%-polyvinyl Alcohol 18.8 parts Water 11.2 parts (Dispersion of Dye Precursor) ODB 2.0 parts Aqueous Solution of 10%-polyvinyl Alcohol 4.6 parts Water 2.6 parts Each dispersion of the above mentioned compounds were ground into fine particles having an average particle diameter of 1 µm (micron) by using a sand grinder. Subsequently, the dispersions were mixed in a following formulation to prepare a coating solution. Dispersion of Developer with Comparative Compound 36.0 parts Dispersion of Dye Precursor 9.2 parts Kaolin Clay (50% dispersion) 12.0 parts Thermal recording materials were produced in the same manner as in Examples 1 to 6. [Control 18] As disclosed in JP-A-5-4449, a zinc salicylate compound was used as the developer, to which the bisthiourea compound was added as a third compound to produce thermal recording materials. (a) 20 g of 2-anilino-3-methyl-6-N-tetrahydrofurfurylamino)fluoran as the dye precursor, (b) 20 g of 4- p -methoxyphenoxyethoxy zinc salicylate (represented by SA1-Zn in Table 3) as the developer, (c) 20 g of the bisthiourea compound E-10 as the additive, and (d) 20 g of di( p -methylbenzyl) oxalate ester as the sensitizer were each dispersed along with 100 g of 5% polyvinyl alcohol (PVA-150 available from Kuraray Co., Ltd.) aqueous solution by using a ball mill over day and night until an average particle diameter of 1.5 µm or smaller was achieved to prepare the dispersions. In addition, 80 g of calcium carbonate was dispersed along with 160 g of 0.5% solution of sodium hexamethacrylate by using a homogenizer to prepare a pigment dispersion (e). The dispersions prepared in the manner described above were mixed in a following formulation to obtain a thermal coating solution. (a) Dye Precursor Dispersion 5 parts (b) Developer Dispersion 10 parts (c) Thiourea Compound Dispersion 3 parts (d) Sensitizer Dispersion 10 parts (e) Calcium Carbonate Dispersion 5 parts The thermal coating solution was applied to a wood free paper of 50 g/m 2 in basic weight by using a wire bar to provide a dry weight of 5 g/m 2 of the coated layer, which was dried at 50°C for 1 minute to produce a thermal recording paper. [Control 19] Control 18 was repeated to produce a thermal recording material except that the 4- p -methoxyphenoxyethoxy zinc salicylate (SA1-Zn) developer was replaced by 3,5-bis(methylbenzyl) zinc salicylate (represented by SA2-Zn in Table 3), and that the compound E-12 was used as the additive in place of the bisthiourea compound E-10. Following evaluation tests were performed on the thermal recording materials produced. Recordability Test (Density of Dynamically Developed Color) A printer of a word processor (RUPO-90F; available from Toshiba Corporation) was used to record images with the maximum applied energy. The recorded images were measured in density by using a Macbeth densitometer (RD-914 with an amber filter; density measurements described below were all obtained under this condition). In this event, the larger a Macbeth value, the thicker the recording density and hence the higher the recording aptitude. Heat-resistance Test A (with Heat Rolls) The thermal recording materials were forced to a hot plate, which had previously heated to 200°C, at a pressure of 10 g/cm 2 for 5 seconds to cause electrostatic color development. The color-developed thermal recording materials were passed between heat rolls of 160°C at a speed of 30 mm/s. Subsequently, color-developed and background portions of the materials were measured with the Macbeth densitometer. The smaller a difference in colored density of the recorded portion before and after passing between the heat rolls, the higher the thermal stability. In addition, an extremely low thermal stability of the recorded portion means a high possibility of erasure of the recorded images by using the heat rolls. Heat-resistance Test B (with Step Edges) Each of the thermal recording sheets was forced to a hot plate, which had previously heated to 150°C, at a pressure of 8 g/cm 2 for 4 seconds, following which the Macbeth density of the color developed portion was measured to examine the thermal stability of the background. The smaller the Macbeth value, the higher the thermal stability of the background. Thermal Laminating Test The thermal recording materials subjected to dynamic color development were interposed between MS pouch films, which were passed between heat rolls of a simple laminating machine (MS Pouch H-140 available from Meiko Shokai Co., Ltd.) at an intermediate speed to thermally laminate the films. The Macbeth densities of the recorded portion and the background were then measured. The smaller a difference in density between the recorded portion and the background before and after the laminating process, the more the material is suitable to be formed into a laminated card. Oil Based Ink Aptitude Test Characters were written on the thermal recording materials with an oil based red ink No. 500 (manufactured by Teranishi Chemical Industry Co., Ltd.). A degree of change in color was measured visually relative to the original red. o ○ no color change ○ less color change ▵ slight color change X remarkable color change Evaluation results of the above mentioned tests on Examples 1-6 and Controls 1-19 are set forth in Tables 1 to 3. As apparent from Tables 1 to 3, in the thermal recording materials comprising the bisthiourea compound according to the present invention as the developer and comprising no sensitizer, the background is substantially unchanged at a temperature of from 120° to 150°C. With a thermal head, however, images having the desired density can be obtained. Accordingly, effects of the present invention are as follows: (1) thermal recording materials become available under a high temperature conditions ranging from 100° to 150°C, which was not suitable for conventional thermal recording; (2) a card having a thermal recorded portion can be readily made since it is possible to laminate a film thermally on the recorded surface after thermal recording; (3) the materials are applicable in various electrophotographic copying machines; (4) the materials cannot be affected by oil based inks, so that any images can be written thereon with these inks; and (5) advanced recording system is achieved in which color recording and erasure can be made repeatedly, which permits recycled use of the thermal recording materials, saving resources. Unlike liquid crystals, the present materials can be used as a simple way of indication for recording and erasing using energies in different levels.

Claims: A thermal recording material comprising a support coated with a colour developing layer, wherein the colour developing layer comprises: (i) a developer which comprises a bisthiourea compound of the following general formula (I): wherein each of X, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 , which may be the same or different, is a lower alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, a cyclohexyl group, a nitro group, a cyano group, a halogen atom or a hydrogen atom; Y is S or SO 2 ; and m is an integer of from 1 to 3; and (ii) a colourless dye precursor with which the developer reacts upon heating to form a colour. A material according to claim 1 wherein the compound of general formula (I) is of one of the following B-1, B-8, and C-1: A material according to claim 1 or 2 wherein the dye precursor is 3-N,N-dimethylamino-6-methyl-7-anilinofluoran. A process for preparing a thermal recorded material, which process comprises subjecting a material as claimed in any one of the preceding claims to recording, to provide a recorded portion on one surface thereof, and applying a plastic film to the surface of the material. A process according to claim 4, which comprises applying the plastic film to the surface by thermal laminating. A thermal recorded card which comprises a material as claimed in any one of claims 1 to 3, the material having been subjected to recording to provide a recorded portion on one surface thereof, and a plastic surface film.

Inventors: HAYASAKA HIDEKI, NAGAI TOMOAKI, SATAKE TOSHIMI, SEKINE AKIO, TAKANO TOSHIYUKI, HAYASAKA, HIDEKI, NAGAI, TOMOAKI, SATAKE, TOSHIMI, SEKINE, AKIO, TAKANO, TOSHIYUKI, HAYASAKA, HIDEKI, C/O CENTRAL RESEARCH LAB., NAGAI, TOMOAKI, C/O RESEARCH LAB. OF PRODUCT DEV., SATAKE, TOSHIMI, C/O CENTRAL RESEARCH LAB., SEKINE, AKIO, C/O RESEARCH LAB. OF PRODUCT DEV., TAKANO, TOSHIYUKI, C/O CENTRAL RESEARCH LAB.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1994
Main classification: B41M5
IPC codes: B41M5